The present invention relates generally to semiconductors and more specifically to an improved fabrication process for making semiconductor memory devices.
Flash electrically erasable programmable read only memory (EEPROM) is a class of nonvolatile memory devices that are programmed by hot electron injection and erased by Fowler-Nordheim tunneling.
Each memory cell is formed on a semiconductor substrate (i.e., a silicon die or chip), having a heavily doped drain region and a source region embedded therein. The source region further contains a lightly doped deeply diffused region and a more heavily doped shallow diffused region embedded into the substrate. A channel region separates the drain region and the source region. The memory cell further includes a multi-layer structure, commonly referred to as a xe2x80x9cstacked gatexe2x80x9d structure or word line. The multi-layer stacked gate structure typically includes: a thin gate dielectric or tunnel oxide layer formed on the surface of substrate overlying the channel region; a polysilicon floating gate overlying the tunnel oxide; an interpoly dielectric layer overlying the floating gate; and a polysilicon control gate overlying the interpoly dielectric layer. Additional layers, such as a silicide layer (deposited on the control gate), a poly cap layer (deposited on the gate silicide layer), and a silicon oxynitride layer (deposited on the poly cap layer) may be formed over the control gate. A plurality of Flash EEPROM cells may be formed on a single substrate.
A Flash EEPROM also includes peripheral portions, which typically include input/output circuitry for selectively addressing individual memory cells.
The process of forming Flash EEPROM cells is well known and widely practiced throughout the semiconductor industry. After the formation of the memory cells, electrical connections, commonly known as xe2x80x9ccontactsxe2x80x9d, must be made to connect the stack gated structure, the source region and the drain regions to other parts of the chip. The contact process starts with the formation of sidewall spacers around the multi-layer stacked gate structures of each memory cell. A silicidation process is applied to the active region. An etch stop or liner layer, typically a nitride material such as silicon nitride, is then formed over the entire substrate, including the multi-layer stacked gate structure, using conventional techniques, such as chemical vapor deposition (CVD). A dielectric layer, generally of oxide such as such as boro-phospho-tetra-ethyl-ortho silicate (BPTEOS) or borophosphosilicate glass (BPSG), is then deposited over the etch stop layer. A chemical-mechanical planarization (CMP) process is applied to the wafer and wafer-scale planarization is achieved. A layer of photoresist is then placed over the dielectric layer and is photolithographically processed to form the pattern of contact openings. An anisotropic etch is then used to etch out portions of the dielectric layer to form source and drain contact openings in the oxide layer. The contact openings stop at the source and drain regions in the substrate. The photoresist is then stripped, and a conductive material, such as tungsten, is deposited over the dielectric layer and fills the source and drain contact openings to form conductive contacts. The substrate is then subjected to a CMP process, which removes the conductive material above the dielectric layer to form the conductive contacts through a contact CMP process.
For miniaturization, it is desirable to have adjacent word lines as closely together as possible. However, in order to accommodate electrical contacts in the active regions (source and drain) between the stacked gates and avoid electrical shorts between stacked gates and core active areas, wide spacing (separation) between word lines is required. This process significantly increases semiconductor memory core cell size and therefore adversely impacts semiconductor device and memory densities. Moreover, this problem is becoming more critical as separation between adjacent stacked gate structures diminishes with semiconductor technology feature size scaling down to sub-quarter micron level and below.
The above becomes worse at smaller geometries because the core region must be treated differently from the peripheral region. In the core region, it is necessary that the gate contact and source/drain contacts be isolated. In the peripheral region, it is necessary that the gate contacts and source/drain contacts be in contact and form local interconnect to increase packing density and device performance.
A solution, which would allow further miniaturization of memory device without adversely affecting device performance has long been sought, but has eluded those skilled in the art. As the demand for higher performance devices and miniaturization continues at a rapid pace in the field of semiconductor, it is becoming more pressing that a solution be found.
The present invention provides a method for shrinking a semiconductor device by processing the peripheral region to form contacts separately in a decoupled process from the core region so the multi-layer stacked gate structures can be positioned closer together.
The present invention provides a method of manufacturing a semiconductor device in which multi-layer structures are formed on a semiconductor substrate to form core and peripheral regions. Sidewall spacers are formed around the multi-layer structures and source and drain regions are implanted adjacent the sidewall spacers and a salicidation process is applied to form source/drain silicides. A stop layer is deposited over the semiconductor substrate and the gate after which a dielectric layer is deposited over the stop layer. A CMP process is applied to the wafer and wafer-scale planarization is achieved. A photoresist contact mask is deposited, processed, and used to form core self-aligned contacts while covering the peripheral region so as to expose the multi-layer structures in addition to the source and drain regions. A second photoresist contact mask is deposited, processed, and used to form peripheral local interconnect gate openings over the peripheral region while covering the core region. A conductive material is deposited over the dielectric layer and in the core contact and peripheral local interconnect openings and is chemical-mechanical planarized to remove the conductive material over the dielectric layer so the conductive material is left isolated in the core and peripheral contact openings with core contacts to the source/drain regions and peripheral contacts to the multi-layer structures and the source/drain regions.